In known circuits, the back-up battery is commonly coupled to the power supply by way of simple isolation diodes or series transistor switches. Both of these switching techniques are relatively inefficient and also the use of isolation diodes is not possible at operating voltages significantly below 2V.
An object of the invention is to provide an improved power switching circuit.
A further object of the invention is to provide a power switching circuit for controlling the supply of power from either a primary or secondary battery, and which is capable of switching between primary and secondary batteries at voltages below 2V.
A still further object is to control battery discharge to provide extended operating life.